This invention relates to methods of manufacturing electromagnetic interference (xe2x80x9cEMIxe2x80x9d) shields and the EMI shields produced thereby.
As used herein, the term EMI should be considered to refer generally to both EMI and radio frequency interference (xe2x80x9cRFIxe2x80x9d) emissions, and the term electromagnetic should be considered to refer generally to electromagnetic and radio frequency.
During normal operation, electronic equipment generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can be of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include gaps or seams between adjacent access panels and around doors, effective shielding is difficult to attain, because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.
Specialized EMI gaskets have been developed for use in gaps and around doors to provide a degree of EMI shielding while permitting operation of enclosure doors and access panels. To shield EMI effectively, the gasket should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the gasket is disposed. Conventional metallic gaskets manufactured from copper doped with beryllium are widely employed for EMI shielding due to their high level of electrical conductivity. Due to inherent electrical resistance in the gasket, however, a portion of the electromagnetic field being shielded induces a current in the gasket, requiring that the gasket form a part of an electrically conductive path for passing the induced current flow to ground. Failure to ground the gasket adequately could result in radiation of an electromagnetic field from a side of the gasket opposite the primary EMI field.
In addition to the desirable qualities of high conductivity and grounding capability, EMI gaskets in door applications should be elastically compliant and resilient to compensate for variable gap widths and door operation, yet tough to withstand repeated door closure without failing due to metal fatigue, compression set, or other failure mechanism. EMI gaskets should also be configured to ensure intimate electrical contact with proximate structure while presenting minimal force resistance per unit length to door closure, as the total length of an EMI gasket to shield a large door can readily exceed several meters. It is also desirable that the gasket be resistant to galvanic corrosion which can occur when dissimilar metals are in contact with each other for extended periods of time. Very low resistance and, concomitantly, very high electrical conductivity are becoming required characteristics of EMI gaskets due to increasing shielding requirements. Low cost, ease of manufacture, and ease of installation are also desirable characteristics for achieving broad use and commercial success.
Conventional metallic EMI gaskets, often referred to as copper beryllium finger strips, include a plurality of cantilevered or bridged fingers forming linear slits therebetween. The fingers provide spring and wiping actions when compressed. Other types of EMI gaskets include closed-cell foam sponges having metallic wire mesh knitted thereover or metallized fabric bonded thereto. Metallic wire mesh may also be knitted over silicone tubing. Strips of rolled metallic wire mesh, without foam or tubing inserts, are also employed.
One problem with metallic finger strips is that to ensure a sufficiently low door closure force, the copper finger strips are made from thin stock, for example on the order of about 0.05 mm (0.002 inches) to about 0.15 mm (0.006 inches) in thickness. Accordingly, sizing of the finger strip uninstalled height and the width of the gap in which it is installed should be controlled to ensure adequate electrical contact when installed and loaded, yet prevent plastic deformation and resultant failure of the strip due to overcompression of the fingers. To enhance toughness, beryllium is added to the copper to form an alloy; however, the beryllium adds cost Finger strips are also expensive to manufacture, in part due to the costs associated with procuring and developing tooling for outfitting presses and rolling machines to form the complex contours required. Changes to the design of a finger strip to address production or performance problems require the purchase of new tooling and typically incur development costs associated with establishing a reliable, high yield manufacturing process. Notwithstanding the above limitations, metallic finger strips are commercially accepted and widely used. Once manufacturing has been established, large quantities of finger strips can be made at relatively low cost.
Metallized fabric covered foam gaskets avoid many of the installation and performance disadvantages of finger strips; however, they can be relatively costly to produce due to expensive raw materials. Nonetheless, EMI gaskets manufactured from metallized fabrics having foam cores are increasing in popularity, especially for use in equipment where performance is a primary consideration.
As used herein, the term metallized fabrics include articles having one or more metal coatings disposed on woven, nonwoven, or open mesh carrier backings or substrates and equivalents thereof. See, for example, U.S. Pat. No. 4,900,618 issued to O""Connor et al., U.S. Pat. No. 4,910,072 issued to Morgan et al.; U.S. Pat. No. 5,075,037 issued to Morgan et al., and U.S. Pat. No. 5,393,928 issued to Cribb et al., the disclosures of which are herein incorporated by reference in their entirety. Metallized fabrics are commercially available in a variety of metal and fabric carrier backing combinations. For example, pure copper on a nylon carrier, nickel-copper alloy on a nylon carrier, and pure nickel on a polyester mesh carrier are available under the registered trademark Flectron(copyright) metallized materials from Advanced Performance Materials located in St. Louis, Mo. An aluminum foil on a polyester mesh carrier is available from Neptco, located in Pawtucket, R.I.
The choice of metal is guided, in part, by installation conditions of the EMI shield. For example, a particular metal might be chosen due to the composition of abutting body metal in the enclosure to avoid galvanic corrosion of the EMI shield, which could increase electrical resistance and deteriorate electrical grounding performance. Metallized tapes are desirable both for ease of application as well as durability.
Metallized fabrics, such as those described in the O""Connor et al. patent mentioned hereinabove, are generally made by electroless plating processes, such as electroless deposition of copper or other suitable metal on a catalyzed fiber or film substrate. Thereafter one or more additional layers of metal, such as nickel, may be electrolessly or electrolytically deposited on the copper. These additional layers are applied to prevent the underlying copper layer from corroding, which would increase the resistance and thereby decrease the electrical conductivity and performance of any EMI gasket made therefrom. The additional nickel layer on the copper also provides a harder surface than the base copper.
Two developments have been progressing independently for several years in the area of EMI shields for nonconductive enclosures, such as molded plastic housings for cellular telephones, computers, and the like. The first development is a form in place (xe2x80x9cFIPxe2x80x9d) process. See, for example, U.S. Pat. No. 5,822,729 entitled Process for Producing a Casing Providing a Screen Against Electromagnetic Radiation, the disclosure of which is incorporated herein by reference in its entirety. A goal of the FIP process is to produce a conductive and compressible elastomeric EMI gasket that can be directly applied to the substrate to be shielded, thereby eliminating the step of attaching the EMI gasket to the workpiece at the assembly plant. One problem with the FIP process is that it is necessary to have relatively complex and expensive dispensing equipment at the casting or molding plant, or at the assembly plant. As the capacity utilization of this equipment may be quite low, due to the use on a single component this is a risky and potentially uneconomic situation.
The second development in the area of EMI shielding is the production of conductive coatings, especially an extensible conductive coating, which is a coating with high conductivity that can be applied to a film, or other flexible substrate, that is later formed to a desired shape without substantial degradation of conductivity. See, for example, U.S. Pat. No. 5,286,415 entitled Water-Based Polymer Thick Film Conductive Ink and U.S. Pat. No. 5,389,403 entitled Water-based Polymer Thick Conductive Ink, the disclosures of which are incorporated herein by reference in their entirety. Acheson Colloids Company, located at Port Huron, Mich., has developed a product based on silver ink that when coated on a thermoformable film, such as General Electric""s Lexan(copyright), retains high electrical conductivity even when drawn to relatively high elongations. The thermoformable film may be formed to relatively complex three dimensional shapes known as xe2x80x9ccans.xe2x80x9d The thermoformable film with extensible coating can replace conventional metal cans, as well as conductive painting and plating processes, used in mobile phones and other nonconductive small enclosures. The thermoformable film and extensible coating can also be part of larger electronic packages.
It has been discovered that thermoformable films, extensible conductive coatings and FIP gaskets can be combined to produce integral EMI shields which can be readily manufactured and shipped from a centralized location to smaller assembly plants for installation into electronic equipment.
The EMI shield is manufactured from a polymer thick film extensible conductive coating, that retains high electrical conductivity at high elongations, which is applied to a thermoformable film in combination with a FIP gasket. The EMI shield and FIP gasket provide EMI shielding of the entire interior of a given structure.
For example, suitable thermoformable films include LEXAN(copyright) and VALOX(copyright), manufactured by the General Electric Company, Pittsfield, Mass. An example of a polymer thick film extensible conductive coating is Electrodag(copyright) SP-405, manufactured by Acheson Colloids Company, Port Huron, Mich.
Accordingly, in accordance with one embodiment, the invention is drawn to a method for forming an EMI shield. The method includes the steps of (a) providing a thermoformable film having a first side and a second side; (b) applying an extensible conductive coating to the thermoformable film; (c) cutting the thermoformable film; (d) thermoforming the thermoformable film into a three-dimensional shape; and (e) applying a compressible EMI gasket to the thermoformable film, wherein steps (b) through (e) may be performed in any order.
In another embodiment the invention is drawn to an EMI shield. The EMI shield includes a thermoformable film having a first side and a second side, wherein the thermoformable film is thermoformed into a three-dimensional shape; an extensible conductive coating applied to the thermoformable film; and a compressible EMI gasket attached to the thermoformable film.
In yet another embodiment, the extensible conductive coating includes an extensible film and conductive fibers. In another embodiment the glass transition temperature of the extensible film is lower than the glass transition temperature of the thermoformable film.